This invention relates to a method for molding a bulge which may eliminate a step for removing air from within a bulge to be molded before molding.
In such a conventional method a cylindrical blank is disposed in a mold of desired shape and a high-speed flow of liquid produced by an impact fluid pressure is jetted into the blank, thereby obtaining a desired shape (Japanese patent publication 23935/1974). Since this method is not required to remove air from within the article before molding, the manufacturing step is simpler and the molding time can be much reduced, thus making is suitable for mass production. Air, if not removed, is locally compressed by such jet flow of pressure liquid and pressure is not sufficiently transmitted thereto, resulting in local deformation of the article. This method can remove such drawback. However, this method requires an exclusive impact liquid pressure generator. Such generator is bulkier and expensive and thus molding apparatus is bulkier as a whole and costly. Furthermore, the blank, if molded under such a momentary high impact pressure, suffers an excessive strain, resulting in a poor yield. For this reason, only a simple form of article can be manufactured. Such conventional method requires a guide for causing a highspeed jet flow of pressure fluid to effectively impinge onto the inner surface of the blank and it also requires a complicated nozzle shape. Since, in this case, the guide must be placed in the mold, the shape of an article to be molded is restricted.
It is accordingly an object of this invention to provide a method for manufacturing a bulge which can save the time of beforehand removing air from within a blank to be molded and which can manufacture a bulge, simple or complicated in design, using small-sized, low-cost apparatus.